What is Variables in Ruby on Rails?

• Variables are identifiers.
• They are storage points that hold known or unknown quantity of information referred to as a value.
• The variable's name should denote the information the variable contains.
• The operations on variables vary for different programming languages and the value they hold.
• In Ruby, for instance, if variable switches to store a boolean instead of an array, the possible operations would vary.
• Still, on variables, we can liken them to Buckets where information can be kept.
• The name of the variables can be seen as name tags placed in the bucket.
• When referring to one of the buckets, we use the name of the bucket, not the data stored in the bucket.

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Scopes of Variables

• There are generally, two scopes of variables. They are:
• Local Variables
• Global Variables

1. Local Variables

• These are variables that are assigned within a method.
• They can be used only within the method in which they have been declared.

def counter
    count = 0
    while count <= 10
        p count
        count += 1
    end
end

count #=> 0 1 2 3 4 5 6 7 8 9 10

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• The example above is a Ruby method I named counter.
• A local variable count was declared in this method and it was used to print 0..10.
• However, if you call countoutside this method, it would throw an undefined variable or method error.
• It is important to note that arguments passed into a method or function, are also treated as local variables.
• For example, the code snippet below works exactly as the one above.

def counter(count)
    while count <= 10
        p count
        count += 1
    end
end
counter(0); #=>0 1 2 3 4 5 6 7 8 9 10

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2. Global Variables

• These are variables that can be used by two or more methods.
• These variables are declared outside the methods and they can be manipulated within methods.
• Global variables in Ruby are denoted with the $ symbol.
• The example below shows how global variables are declared and used in Ruby.

$count = 0
def counter
    while $count <= 10
        p $count
        $count += 1
    end
end
counter #=>0 1 2 3 4 5 6 7 8 9 10

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• · You will notice that count was declared outside the method, and it was incremented within the method.
• Also, we didn't need to pass count as an argument when calling the method.

Variables in Ruby

• Based on these two scopes of variables, Ruby has different types of variables. They are:
• Instance Variables
• Class Variables
• Class Instance Variables

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1. Instance Variables

• Instance Variables are global variables that are ONLY used in Instance Methods within a class.
• Instance variables in Ruby start with a single @ sign.
• Any variable declared starting with a single @ sign can be manipulated within instance methods in Ruby.
• The example below shows an instance variable than has been used in an instance method.

class User
    def initialize
        @name = "John"
    end
    def greet
        "Hello #{@name}"
    end
end
User.new.greet #=> “Hello John”

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• The above example has instance variable @name which holds a name John.
• The instance method greet adds Hello to the value of @name.
• This shows that instance variables act as global variables within a class.

2. Class Variables

• Class variables work exactly like instance variables.
• These variables are denoted with a double @ sign, i.e @@.
• These variables are used in both Instance Methods and Class Methods within a class.
• They can be used anywhere within the class or its children.
• They are the closest to the regular global variable in Ruby.
• For example, if we were to rewrite the above example as a class method, the method greet will be written as self.greet.
• Class methods, however, can be called directly on the class they belong to.
• The example below shows how a class variable has been used in a class method.

class User
    @@name = “John”
    def self.greet
      "Hello #{@@name}"
    end
end
User.greet  #=> "Hello John"

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3. Class Instance Variables

• These variables are a combination of Instance variables and class variables.
• This means they are declared in a class method like instance variables, but they can be used in any other class methods within the same class and other classes that inherit from that class.
• The example below shows how class instance variables are used.

class Person
    def self.username(name)
      @name = name
    end
    def self.greet
      "Hello #{@name}"
    end
end
Person.username("John") #=> "John"
Person.greet #=> "Hello John"

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• In the snippet above, name method was used to get the person's name and it was assigned to @name variable.
• This variable, however, can be used in any other class method within that class.
• One important thing to know is that these kind of variables will not be available in classes that inherit from the class in which they are declared.

class Person
   @name = "John"
   def self.greet
       "Hello #{@name}"
    end
 end

Person.greet #=> "Hello John"
class Student < Person
end
Student.greet #=> "Hello "
class Student < Person
  @name = "James"
end
Student.greet #=> "Hello James"

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• From the example above, @name was assigned to "John" in class Person and calling Person.greet returned "Hello John"
• But that was not the case in class Student. class Student inherits everything from class Person except the value of @name.
• In other to use this variable in this new class, the variable has to be declared again.

What makes these variables different from each other?

• The major differences between these variables wrap around the scope at which they are to be used and the syntax for assigning them.
• The difference between an instance variable and a class instance variable is the scope at which they are used.
• Instance Variables are scoped to Instance Methods while Class Instance Variable are scoped to Class Methods.
• The difference between a Class Instance Variable and a Class Variable is the syntax of assigning them and their behavior could also count.
• Class instance Variables are assigned with a single @ symbol while Class Variables are assigned with double @ symbol, i.e @@

Conclusion

• Variables generally work almost the same way across all computer programming languages.
• What differentiates them is the way they are used, how they are been assigned, and the syntax used to declare them.

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